Image forming apparatuses are known in which, at the time of replenishing developer during a toner exchanging operation or in the case where images with a high print ratio have been printed continuously, toner corresponding to the consumed toner is replenished from a toner cartridge so as to keep the ratio of the toner to the carrier in the developer constant. However, when the amount of the toner to be replenished is large, there is a problem that the carrier and the toner are not agitated sufficiently, and thereby the sufficient charging amount of toner can not be obtained.